Datasheet

Data Sheet AD7877
Rev. D | Page 25 of 44
If f
DCLK
= 20 MHz (maximum), then t
DCLK
= 50 ns.
t
2
= 512 µs with timer set to 1024 (TMR bits = 01b)
t
WRITE
= t
READ
= 16 CLK period × t
DCLK
= 800 ns
t
3
= maximum time taken to write read address and read
11 registers = 800 ns (write) + [800 ns (read) × 11] = 9.6 µs.
t
4MAX
= t
2
− t
3
= 512 µs − 9.6 µs = 502.4 µs
Pen Interrupt (
PENIRQ
)
The pen interrupt request output (
PENIRQ
) goes low whenever
the screen is touched. The pen interrupt equivalent output
circuitry is outlined in Figure 43. This is a digital logic output
with an internal pull-up resistor of 50 kΩ, which means it does
not need an external pull-up. The
PENIRQ
output idles high.
The
PENIRQ
circuitry is always enabled, except during
conversions.
03796-021
X+
TOUCH
SCREEN
Y+
50kΩ
Y–
X–
PENIRQ
ENABLE
PENIRQ
V
CC
V
CC
Figure 43.
PENIRQ
Output Equivalent Circuit
When the screen is touched,
PENIRQ
goes low. This generates
an interrupt request to the host. When the screen touch ends,
and if the ADC is idle,
PENIRQ
immediately goes high. If the
ADC is converting,
PENIRQ
goes high once the ADC becomes
idle. The
PENIRQ
operation for these two conditions is shown
in
Figure 44.
03796-022
SCREEN
PENIRQ
ADC
STATUS
TOUCHED
NOT
TOUCHED
NOT
TOUCHED
NOT
TOUCHED
NOT
TOUCHED
ADC IDLE
SCREEN
PENIRQ
ADC
STATUS
TOUCHED
ADC IDLE
ADC
CONVERTING
ADC IDLE
RELEASE NOT
DETECTED
PENIRQ
DETECTS
RELEASE
PENIRQ
DETECTS
RELEASE
PENIRQ
DETECTS
TOUCH
PENIRQ
DETECTS
TOUCH
Figure 44.
PENIRQ
Operation for ADC Idle and ADC Converting
SYNCRONIZING THE AD7877 TO THE HOST CPU
The two suggested methods for synchronizing the AD7877
to its host CPU are slave mode, in which the mode bits can
be either 01b or 10b, and master mode, in which the mode
bits are 11b.
In slave mode,
PENIRQ
can be used as an interrupt to the host.
When
PENIRQ
goes low to indicate that the screen has been
touched, the host is awakened. The host can then program the
AD7877 to begin converting in either Mode 01b or Mode 10b,
and can read the result registers after the conversions are
completed.
In master mode,
DAV
can also be used as an interrupt to the
host. However, the host should first initialize the AD7877 in
Mode 11b. The host can then go into sleep mode to conserve
power. The wake-up on touch feature of the AD7877 is active in
this mode, therefore, when the screen is touched, the
programmed sequence of conversions begins automatically.
When the
DAV
signal asserts, the host reads the new data avail-
able in the AD7877 results registers and returns to sleep mode.
This method can significantly reduce the load on the host.